This invention relates generally to heat exchanger coils and, more particularly, to a method and apparatus for the tension expanding of plate fin heat exchanger coils one hairpin at a time.
A plate fin heat exchanger coil is commonly constructed with a plurality of flat, parallel plates having laterally spaced holes therein for receiving refrigerant tubes, or hairpin tubes, therein. At each end of the plate fin bundle, there is generally a tube sheet composed of heavier material, and adjacent the upper tube sheet, the open ends of the hairpin tubes are fluidly connected by way of U-shaped return bends that are secured thereto by way of soldering or the like. When the coils are installed into a refrigeration system, the refrigerant is made to flow through the hairpin tubes, and the air to be cooled or heated is made to flow over the plate fins, such that a heat transfer is thereby affected.
As is known, the tube compression expansion process is generally carried out in large presses, in which all the tubes are expanded at one time, by passing tube expanding rods through the open ends of the hairpin tubes and then belling the tube. A backing plate is placed against the hairpin tube bends during compressive expansion to prevent the tubes from being driven out of the unit as the expanding tools are forced therethrough. As a result of this holding action tubes are compressed rearwardly as they are being expanded outwardly by the tools. This in turn, causes the tubes to change length so that the axial length of each tube can vary dramatically in final assembly. Because of the differences in tube length, belling of the tubes is difficult and generally results in uneven or misaligned bells being formed in the tube ends. The return bend therefore cannot be properly seated within the bells leading to the formation of relatively weak or incomplete solder joints in this critical region.
In U.S. Pat. No. 4,584,765 issued to Gray, there is disclosed a technique for tension expanding hairpin tubes into a fin pack unit. This prior technique was generally carried out on a three row coil which was first belled by a split collet and pin arrangement, and then all of the tubes were expanded at one time. Because of the closeness of the heat exchanger tube rows the bells that were formed are simple, single diameter bells, to which pre-tinned return bends are used. Further, because of the closeness of the heat exchanger tube rows, the outer jaw members of the tube clamping fixture were massive while the inner jaw member were relatively long, thin members. In operation, however, these long, thin jaw members bowed or bent in the transverse direction to the direction of the movement of the expanding bullet during the expansion process due to the large forces necessary to expand the coil. Furthermore, the single diameter bell was not suitable for ultrasonic soldering, but had to use pretinned return bends. Thus, to use ultrasonic soldering, the bell diameter would have had to be increased, which would have required even less space between adjacent tubes, making the jaw member even thinner and subject to more bowing.
A further disadvantage of the prior compression or expanding devices is that they were not very flexible in manufacturing coils of different configurations. As a consequence, the manufacturing of a variety of different coils is time consuming and costly due to the set-up time necessary to change expansion rods and hairpin receiver blocks when changing from one coil configuration to another coil configuration.
Thus, there is a clear need for a tension expander that can be used to tension expand many different configurations of plate fin heat exchanges without gripping the tubes as they are expended.